1. Field of the Invention
The present invention concerns a magnetic resonance system and an operating method therefor of the type wherein magnetic resonance data are acquired according to a SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions) or equivalent pulse sequence, and in particular to such a system and method for flow artifact reduction in slab selective SPACE imaging.
2. Description of the Prior Art
Highly sophisticated spin-echo pulse sequences include a single-slab 3D turbo or fast spin-echo pulse sequence known, for example, as SPACE. Pulse sequences of this type allow an extremely large number of refocusing RF pulses (such as more than 300) by using a refocusing RF pulse train exhibiting pulses with respectively different flip angles throughout the duration of the echo train. The curve that represents the variation of the flip angles is designed to achieve desired signal strengths for different types of tissue (nuclear spins), and is referred to as the flip angle evolution. Such an evolution is usually designed for obtaining a specific contrast (such as in proton density-weighted images or T1-weighted images or T2-weighted images) between the tissues in the image. Such an imaging sequence can be used effectively in brain imaging, for example, wherein cerebral-spinal fluid (CSF), gray matter and white matter all exhibit markedly different signal intensities in T2-weighted images. Using the SPACE sequence, an optimal T2-weighted contrast among the various tissues can be obtained by setting the echo time around the middle portion of the echo train.
A basic description of single slab SPACE imaging can found, for example, in U.S. Pat. No. 7,705,597 and in the article “Fat-Signal Suppression in Single-Slab 3D TSE (SPACE) Using Water-Selective Refocusing,” Mugler, III et al., Proc. Intl. Soc. Mag. Reson. Med., Vol. 19 (2011), page 2818.
In slab-selective SPACE imaging, flow-related artifacts often occur in the readout direction, for example, in spine imaging due to the CSF flow. An example of such a flow-related artifact can be seen in FIG. 1, in the outlined region. In contrast to the phase-encoding direction, this problem primarily occurs in the readout direction, because this direction is more sensitive to flow.
In an effort to address this problem, imaging sequence protocols are configured with the phase-encoding direction being aligned with the cranio-caudal or head-to-feet axis of the patient. This means, however, that a large number of phase-encoding steps are needed to cover the field of view (FOV) of interest, and a large number of phase-encoding steps are necessary for oversampling (normally about 50% to 80%), in order to avoid infolding artifacts. These factors result in a very long acquisition time when such imaging sequences are used. This situation is illustrated in FIG. 2, which shows a portion of the pulse-sequence elements for a conventional single slab SPACE imaging sequence in which the RF excitation pulse is shown at the top left, followed by the refocusing RF pulses of varying amplitude (labeled “RF signal”). The sequence for the X gradient is shown below, with a relatively long time duration between the dephasing gradient and the first application of a readout gradient, during which gradient data acquisition may occur. For completeness, the Z gradient is shown in FIG. 2 as well, below the X gradient.